Transmission circuits



Oct. 30, 1923. 1,472,610

R. C. MATHES TRANSMISS ION CIRCUITS Filed Dec. 4, 1919 hvepfor: 2w? c? Mai/7e5- Wmy Patented Oct. 30, 1923.

UNITED STATES PATENT OFFICE.

ROBERT C. MATHES, OF NEW YORK, N. "1., ASSIGNOR TO WESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION OF NEW YORK.

TRANSMISSION CIRCUITS.

To all whom it may concern:

Be it known that I, ROBERT C. .lsiarrrns, a citizen of the UnitedStates, residing at New York, in the county of Bronx, State of New York, have invented certain new and useful Improvements in Transmission Circuits, of which the following is a full, clear, concise, and exact description.

This invention relates to transmission circuits and more particularly it relates to twowa repeating systems.

s is well-known in the art, it is fre quently found desirable to provide a repeating or amplifying system between two transmlssion lines. In one form, such a repeating system may have the transmission lines terminate in a Wheatstone bridge type of circuit for-securing two-way transmission from a unidirectional amplifying system.

As is well-known the balanced condition of such bridge circuits is maintained by providing networks which closely similate the impedance of the transmission lines, or by balancing two similar transmission lines against each other. It is further well-known in the art that if a high degree of balance is not obtained, there is a critical amount of amplification, beyond which onecannot go without setting up a local circulation of 0 energy in the form of a sustained tone, commonly known as singing. It is evident, therefore, that if any condition exists, other than the unbalance between the line and the network, which causes the transmission of electrical energy between the two circuits designed to be con1ugate, such a conditlon will further limit the amount of gain which,

can be obtained from'the set.

It has been found in particular arrangements of telephone repeater equipment that the capacities between the primary and the secondary windin s of the transformer coil aiford a path for File transmission of energy between the parts of the circuits presumably balanced. As the transmission of such undesired potentials isfrom a series condenser it is most efiicicnt at high frequencies so that it may then result in a sustained singing at an inaudible frequency. This result, while not interfering as adirect tone, seriously impairs the normal telephone transmission of the circuit and so necessitates a reduction in amplification.

In accord nce with this invention this ob Serial 1T0. 342,322.

jectionable loss in transmission efiiciency of a two-way repeater circuit may be'overcome by shunting these undesired potentials through a low impedance path to ground. One way this may be accomplished is by lnserting grounded shields between the primary and secondary windings of one or more of the transformers located in the repeater circuit.

This invention will be better understood by reference to the following detailed description taken in connection with the accompanying drawing, in which Fig. 1 represents this invention embodied in a two way two repeater system; Fig. 2, is a modification of Fig. 1 and Fig. 3 represents this invention embodied in a two way one repeater system. Referring to Fig. 1 of the drawing, 3 and 4 are two transmission lines between which is located a two-way repeating system. Line 3 terminates in a bridged circuit containing a balancing network 5 of such a character as to similate the impedance of the line. 6 is a similar balancing network for line 4. Between line 3 and network 5 are the four equal windings 7, 8, 9 and 10 of a repeating coil 11. Incoming currents from line 3 by connections across the bridge points 12 and 13 are impressed through a filter 14, upon an electrical wave amplifier 15 of a suitable type. The amplified currents of the output circuit of amplifier 15 by repeating coil 16, are impressed upon outgoing line 4 in a manner now well-known in the art without any appreciable part being diverted back to the input of amplifier 20. Similarly currents from line 4 by the connections from bridge points 17 and 18 of repeating coil 16 are impressed through a wave filter 19, upon an amplifier or repeater 20. .The amplified currents in the output circuit of amplifier 20 are impressed through repeating coil 11, upon the outgoing line 3. The balancing networks?) and 6 for lines 3 and 4 respectively, may be of any suitable design. For example, they may be of the form disclosed in the Hoyt Patent No. 1,167 ,694 of August 7 1915. The preferred form of filters 14 and 19 is disclosed in the Campbell Patent No. 1,227,114 of May 22, 1917. As described in this Campbell patent, these filters serve to prevent the repeaters from singing due to any feed back of energy caused by an unbalance between a line and its balancing network for currents of frequencies higher than the range of frequencies it is desired to transmit through the amplifier.

As pointed out above a local circulation of power in the two halves of the repeater circuit, other than that due to improper line balance, is liable to occur due to capacities between the windings of the transformers in the circuit. For example, if the control electrode 23 of amplifier 15, were to undergo a change in potential, this change would be amplified by amplifier 15 so that the winding 241; of repeating coil 16 would acquire a certain potential to ground. Due to the capacity present between winding 2% and the other windings of repeating coil 16, all the apparatus between repeating coil 16 and the primary of transformer 34, would, therefore, acquire a certain potential with respect to ground; The potential with re spect to ground which the primary of transformer 34 thus has, will be transmitted through the capacity between the primary and secondary windings of the transformer so as to impress a potential on the control electrode of amplifier 20. This amplified potential, due to the capacity effect between the primary winding and the secondary winding of transformer 11 will give all the apparatus located between the secondary of the transformer 11 and the primary of transformer 33 a potential with respect to ground. This potential in turn will be transmittedthrough the capacity of transformer 33 to give a potential to the control electrode of amplifier 15 which potential will again be amplified amplifier 15. It is evident that if the returned impulse is equal to the initial impulse, sustained singing will be set up. Even if an insufficient amount is fed back in this way to produce the grounded connection of the cathodes of the repeaters. The secondary of each output transformer together with. each intermediate transformer 36 or 37, the filter and the, primary winding of each input transformer are symmetrically located with re.

pe t9 site dl het s' h p ten t ground impressed upon the secondary of each output, transformer does not create a CulI8I1ll,Wl'llCl1, flOWS, serially through the l two sidelco nductors 40 and 41, for example,

but the effect transmitted through the ide eei di e r i raifa l- 1. he o s the potential to ground gives rise to longi tudinal currents in this part of the circuit. The manner of the transfer of the potential to ground from the output transformer to the input transformer is, therefore, such that the high frequency singing set up thereby cannot be prevented byadjusting the constants of filters 14 and 19 to cut out the high frequency singing. These filters can be made to cut out the high frequency Singing only when the current of the undesired frequency flows serially through the two side conductors of the circuit.

One way in which this local circulation of power may be overcome is to associate grounded shields with the windings of the transformers that are in that part of the repeater circuit which is symmetrically located with respect to ground. Thus, metallic grounded shields 27 and 28 may be placed between the primary and the secondary windings of transformers '11 and 16. vVith such an arrangement, potentials to ground, which, for example, winding 24 may acquire, will not be impressed upon the secondary winding of transformer 16, due to the capacity between the windings but will find a ready path to ground due to the grounded connection of the shield 27, which is physically interposed between the two windings. T

Similarly if there is a tendency for any potential in the unidirectional path, containing repeater 20, to be fed back through the capacity between the windings of repeating coil 11 to the other unidirectional path, this potential will find a ready path to ground due to the presence of the grounded shield 28 interposed between the primary and secondary windings of transformer 11. The provision'of these two grounded; shields eliminates the transmission of energy from one-half of the repeater circuit to the other, through the capacities in the coil windings.

Another way in which the local circulation of power may be overcome, is by inserting metallic grounded shields 31 and 32 between the primary and secondary windings of input transformers 33 and 34. lVith such an arrangement, any potential from the unidirectional path containing repeater 15, impressed through the capacity between the windings of transformer 16, will not be impressed upon amplifier 20, but will find a ready path to ground due to the presence of the grounded shield 32. Still another way this local circulation of power may be prevented, is by inserting grounded shields 42 and 43 between the primary and secondary windings of intermediate transformers 56v and 37.

Fig. '2, illustrates still another way for preventing this undesired circulation. of power in the two halves of the repeater circuit. Windings 45 and 46 are windings which may be'employed for monitoring purposes in connection with transformer 16, while windings i7 and L8 may be similar monitoring windings for transformer 11. If, now, these four windings are connected through a low impedance path to ground, they will act in a similar manner to the conductive grounded shields. 27 and 28 of Fig. 1, providing they are physically located between primary and secondary windings of the respective transformers.

Several ways have been described above for increasing the transmission efficiency of the two-way repeater system for eliminating effective internal unbalance by providing a symmetrical path to ground for that portion of the repeater apparatus which is floating with respect to ground. In any particular case it may be desirable to employone of the above methods in preference to the others, or, it may be found desirable to employ all of the methods in the same circuit.

Fig. 3, illustrates this invention in connection with a two-way'one repeater system, in which the balancing networks are eliminated and the first transmission line 50 is balanced by the other transmission line 51. Located between the two lines, 50 and 51 are the four windings 52, 53, 54: and 55 of a repeating coil 56, across the bridge points 57 and 58 of these windings, are connections which lead through a filter 59 to the primary winding of input transformer 60. Amplifier 61 has its input terminals connected to the secondary of transformer 60, and has its output terminals connected to the primary winding of repeating coil. 56. With such an arrangement, signals originating at either of the two transmission lines will be impressed through the filter upon the amplifier and the amplified signals will be impressed back on the two transmission lines by repeating coil 56. As in. the repeating system of Fig. 1, the repeating system of Fig. 2 may be free from any circulation of power other than that due to a line unbalance by either inserting a conductive grounded shield 62, between the windings of transformer 56 by inserting a grounded shield 63, between the winding of input transformer 60, or, by inserting a grounded shield 64, between the windings of the intermediate transformer 65.

From the above description it is apparent that applicants invention is not limited to the particular circuit arrangement above described and that it may be variously modified without departing in any wise from the spirit of this invention as we find in the appended claims.

What is claimed is:

1. A two-way repeating system comprising a unidirectional path for repeating in one direction, a second unidirectional path for repeating in the opposite direction and means for substantially eliminating singing comprising a transformer in each of said paths, each of said transformers having grounded shield.

2. A two-way repeating system compris ing a unidirectional path for repeating in one direction, a second unidirectional path for repeating in the opposite direct-ion and means for substantially eliminating singing comprising a transformer in each of said paths, each of said transformers. having a primary winding, a secondary windingland a shield interposed between said windings, said shield being grounded.

3. A two-way repeating system, comprising a unidirectional path for repeating in one direction, a second unidirectional path for repeating in the opposite direction, a repeater and a transformer in each of said paths, each of said transformers having a primary and a, secondary winding and a grounded shield interposed between the windings of one of said transformers for preventing output energy from one of said paths from being fed back through the other said paths to the input terminals of the repeater in said first mentioned path.

4. In combination, two line sections, a two-way repeating system therebetween comprising a unidirectional path for repeating in one direction between said lines,

said path comprising an amplifier, a transformer having a primary and a secondary winding for coupling said amplifier to one of said lines, and grounded means interposed between said primary winding and said secondary winding for preventing output en ergy from said amplifier from being fed back to the input of said amplifier due to longitudinal currents in said path.

5. In combination, two line sections, a two-way repeating system therebetween comprising a uni-directional path for repeating in one direction between said lines, said path comprising an amplifier, a transformer for coupling said amplifier to one of said lines, a. wave filter for preventing said amplifier from singing due to the feedback of a portion of the output of said amplifier into the input of said amplifier by series currents in said path, and grounded means locatedbetween said primary winding and said secondary winding for pre venting said repeater from singing due to longitudinal currents in said path.

6. In combination, two line sections, a two-way repeating system therebetween comprising a uni-directional path for repeating in one direction between said line sections, said pat-h comprising an amplifier, a transformer having a primary and a secondary winding for coupling said amplifier to one of said line sections. a wave filter in said path adjusted to suppress currents of frequencies outside of the range of he quencies to be transmitted between said line sections for preventing said. amplifier from singing due to a feed-back of energy having frequencies outside of the transmission range, and a grounded shield between said primary and said secondary windings for preventing the singing or said amplifier due to longitudinal currents in said path.

7 In combination, two line sections, a two-way repeating system therebetween comprising a uni-directional path for repeating inone direction between said line sections, an artificial line for balancing each of said line sections for the essential range of frequenciesto be transmitted, said path comprising an amplifier, a transformer having afprimary and a secondary winding for coupling said amplifier to one of said line sections, a'wave filter in said path for preventing said amplifier from singing due to any unbalance between one of said lines and its balancing network for frequencies above the essential range to be transmitted, said filter being of such a type as to suppress currents in said path of frequencies beyond the essential range to be transmitted only when said currents are flowing serially-in said path, and a grounded shield between said primary winding and said secondary winding for preventing said amplifier from singing due to longitudinal currents in said path which are freely transmitted through said filter.

In witness whereof, I hereunto subscribe my name this 1st day of December A. 1).. 1919.

ROBERT C. MATHES 

